1. Field of the Invention
The subject invention relates generally to heating apparatus and, more particularly, to an improved heating system particularly suited for portable spa applications.
2. Description of Related Art
Prior art approaches for heating the water of portable spas have typically employed so-called "tubular heaters" enclosed within a large reservoir-like metal enclosure or housing. The metal enclosure provides a current collector, typically a U. L. required function in such applications. Such systems exhibit a number of drawbacks impairing their reliability and maintainability.
Tubular heating design consists of three major components: a metal tube, known as a sheath, an electrical resistance wire placed approximately in the center of the sheath, and magnesium oxide (MgO) electrical insulation which has been shaken and packed inside the sheath. Although the MgO is an electrical insulator, it is a relatively good conductor of thermal energy. This characteristic allows heat generated by the resistance wire to move outward to the sheath, which ultimately heats the surrounding water. The heater components are initially assembled into a long, straight sheath, which is typically bent into various shapes such as coils or "bow ties," the ends of which are then welded to a mounting plate or housing.
Each bend and weld provides an opportunity for failure of the sheath, resistance wire, and MgO. Sheath material stretches, thins, and becomes metallurgically inferior and susceptible to corrosion. The electrical and mechanical properties of the resistance wire degrade. MgO placement and compaction changes, jeopardizing electrical and thermal performance. Over the typical range of operation, heater resistance wire life is approximately inversely proportional to resistance wire temperature or, more simply, lower operating temperature implies longer life.
As noted above, previous spa heater system designs have used large, reservoir-like metal enclosures. These enclosures have caused reliability problems and added significant cost. The major deficiency of these enclosures is the slow water velocity through them. Water velocity is critical to proper cooling of the heater element. Without adequate velocity, the water adjacent to the sheath may actually boil. When this happens, scale and sediment will be left behind which may cause clogging of filters, ozone injectors, and the heater itself. If the heater sheath develops scale, it may burn out due to resistance wire overheating or fail due to corrosion in the scale area.
Servicing of conventional spa heaters can also require draining of the spa and removing components and other cumbersome procedures.